1. Field of the Invention
The present invention relates to a symbol synchronization apparatus and method of a passive radio frequency identification (RFID) reader, which demodulates a tag signal received under a passive RFID environment, and more particularly, to a symbol synchronization apparatus and method of a passive RFID reader, which reliably performs symbol synchronization on a tag signal distorted by transmission energy components and a subcarrier-type tag signal.
2. Description of the Related Art
In general, the RFID technology refers to a technology which reads or writes information from or into a tag with unique identification information in a non-contact manner using a wireless frequency such that an article, an animal, or a person with the tag may be recognized, traced, and managed.
An RFID system includes a plurality of tags or transponders having unique identification information and attached to articles or animals, and an RFID reader or interrogator reading or writing information from or to the tags.
Such an RFID system may be divided into a mutual induction type and an electromagnetic wave type, depending on the mutual communication scheme between the reader and the tags, into an active type and a passive type depending on whether the tags operate with their own power or not, or into a long wavelength type, a medium wavelength type, a short wavelength type, and an ultra-short wavelength type depending on a frequency used by the system.
While receiving a tag signal, the RFID reader communicating with a passive RFID tag should continuously supply transmission energy to the passive RFID tag. Therefore, when the transmission/reception isolation is not sufficiently secured, a large amount of transmission energy leaks into a receiver stage of the RFID reader.
The transmission energy leaking into the receiver stage may cause some data to be lost at or around a section in which the preamble of a received signal starts, and may cause DC-offset noise.
In the receiver stage of the RFID reader which performs general symbol synchronization, it is difficult to accurately synchronize tag signals distorted by such a transmission leakage signal. In particular, since a local peak signal is generated during a matched filter output for a subcarrier-type tag signal, it is not easy to achieve a symbol timing lock.
FIGS. 1A and 1B show a matched filter output for a subcarrier signal (M=2) with a low frequency (LF) of 640 kHz. FIG. 1A is a graph showing the signal level of a symbol 1, and FIG. 1B is a graph showing the signal level of a symbol 0. Referring to FIGS. 1A and 1B, it can be seen that a matched filtering signal of the subcarrier signal contains a local peak signal as well as a peak value indicating a symbol.
FIG. 2 illustrates preamble signals for a subcarrier signal which are described in the ISO 18000-6C standard (UHF Gen2 protocol standard). FIG. 3 shows an example of a subcarrier signal distorted by a transmission leakage signal.
Referring to FIG. 2, when M=2 in the subcarrier signal, two unit pulse signals (bit data) are implemented as one symbol. When M=4, four unit pulse signals (bit data) are implemented as one symbol. When M=8, eight unit pulse signals (bit data) are implemented as one symbol.
A preamble signal includes 4M/LF data in which an identical unit pulse pattern is repeated and ‘010111’ data.
As shown in FIG. 3, however, the start data of the tag signal (that is, the 4M/LF data of the preamble signal) may be partially distorted or lost by a transmission leakage signal under the passive RFID environment.
As described above, the start data of the tag signal may be partially distorted or lost by the transmission leakage signal under the passive RFID environment, and the matched filtering signal for the subcarrier signal has a local peak signal. Therefore, it is not easy to accurately extract and set up a symbol decision time.